In using radiation systems, such as X-ray equipment, as in medical or other contexts, it is desirable to provide systems which can effectively control the amount of X-ray exposure to which an object is subjected, particularly where such equipment is to be used with objects such as human or animal patients. In order to control such exposure, an appropriate determination of the radiation to which the object has been exposed is required and various means can be used for automatically shutting off the equipment when the total exposure reaches a preselected level.
In such systems, suitable X-ray film and one or more phosphorous scintillating sheets (sometimes referred to as intensifier screens) for converting X-rays to photons are often placed within a cassette housing so that the overall cassette package can be positioned behind an object to be X-rayed, for example, at an appropriate position of the anatomy of a patient in alignment with the portion of the anatomy at which the X-rays are to be directed. One or more suitable dectector means are then positioned adjacent to and externally to the film cassette to detect the level of X-ray radiation at such position and to provide an output signal for supply to electronic circuitry where it can be suitably integrated as a function of time so that the integrated output signal provides an integrated measurement thereof which is proportional to the amount of X-ray radiation which has been received by the patient over such integration time interval. When the integrated measurement reaches a preselected level, appropriate circuitry can be used to automatically shut off the X-ray equipment.
One approach to providing such detection operation is to utilize a structure which is separate from and not capable of being fixedly attached to the film cassette and which includes an array of series-connected detectors (e.g., pohotovoltaic cells) individually supported on an insulator substrate. A suitable scintillator sheet, or screen, may also be mounted adjacent the detector cells for amplifying the light photon level for detection by the photovoltaic cells, for example. Such separate device can be fitted with a suitable handle so that it can be positioned adjacent the film cassette during use. An example of such an approach is shown, for example, in U.S. Pat. No. 4,053,774 issued on Oct. 11, 1977 to Berdahl.
A similar approach has been suggested in a system made and sold by Advanced Instrument Development, Inc. of Melrose Park, Illinois under the designation Mobil-AID.RTM.. Such system uses a separate structure in the form of a "paddle" which contains an ion chamber detector placed at a selected location within the paddle structure. The location of the ion chamber field is visually indicated on the face of the paddle and measuring marks are designated along the edges thereof. The paddle is placed over the patient so that the ion chamber field designation (e.g., a circle) is aligned with the portion of the patient's anatomy of interest. The paddle is then removed from above the patient and slid under the patient in alignment with the film cassette using the previously noted measuring marks as a guide to its final positioning.
In both instances the paddle or other separate structure requires a manual alignment of the areas of interest so that the separate detector and the film cassette structures are both in line with the selected anatomical region of the patient. This procedure gives rise to errors in the correct positioning of the separate paddle or cassette devices and, moreover, each time a new film cassette is used, the alignment procedure must be repeated thereby adding to the overall time for setting up the patient for the X-ray process.
Further, if the patient moves during the X-ray process the alignment of the detectors with the film cassette can become disturbed so that the detectors are no longer detecting X-ray radiation at the proper location and in some cases may become completely misaligned with the film cassette. Since the attenuations of the X-ray radiation passing through the film cassette may vary from film cassette to film cassette, the results of the X-ray exposure detection process may also vary so that the integrated output for comparable X-ray radiation dosages will not provide uniform results unless some technique for compensating for such varying attenuations can be devised.
It is desirable that a system be devised which is capable of avoiding such problems in a manner which assures continued alignment of film and detectors during use and provides uniform results from cassette to cassette, all at reasonable cost.